OSL: add basic OSL shader template

Add a basic OSL shader that shows how inputs
and outputs work and do some simple math with
them.

This template is a happy medium between the
templates we already ship, empty_shader is a
little too bare, and the other templates are
a little "too much" and you end up having to
delete a whole bunch of stuff.

a great starting point for some experimentation!

Differential Revision: https://developer.blender.org/D9933

reviewed by: brecht

release/scripts/templates_osl/basic_shader.osl

@@ -0,0 +1,10 @@
+shader basic_shader(
+    float in_float = 1.0,
+    color in_color = color(1.0, 1.0, 1.0),
+    output float out_float = 0.0,
+    output color out_color = color(0.0, 0.0, 0.0)
+    )
+{
+    out_float = in_float * 2.0;
+    out_color = in_color * 2.0;
+}